Consolidation state of incoming sediments to the Nankai Trough subduction zone: Implications for sediment deformation and properties

Abstract

The hydromechanical properties of accreted and underthrust sediments are key parameters controlling the mechanics of earthquakes and the development of fluid pressure in subduction zones. We conducted consolidation tests on sediments from the Philippine Sea Plate (PSP) in the Nankai Trough to understand the consolidation state and hydraulic properties of the incoming sediment section before its incorporation into the subduction zone. We used mudstone and sandstone cores sampled from the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment at two reference sites (Site C0011 located on a basement low; and Site C0012 located on a basement high). Our experimental results indicate that most of the mudstone samples are normally consolidated or overconsolidated, with overconsolidation ratios (OCR) ranging from 0.89 to 2.52 at Site C0011 and 0.86 to 3.85 at Site C0012. Higher OCR values at Site C0012, at least at shallow depths, are likely caused by erosional unloading. This implies that Site C0011 may serve as a better geotechnical reference site. We also find that mudstones accreted along the frontal thrust are severely overconsolidated relative to coeval mudstones at Site C0011, which likely reflects enhanced consolidation due to increased horizontal tectonic stress. Sandstones in the incoming section on the PSP exhibit 2-3 orders of magnitude higher in situ permeability than the mudstones, and the siliciclastic sandstone we tested maintains a high permeability at stresses up to at least 70 MPa, suggesting that the sandstones may act as important pathways for drainage or pore pressure translation from depths of several kilometers.

Coupled Evolution of Deformation, Pore Fluid Pressure, and Fluid Flow in Shallow Subduction Forearcs

“…(a) Consolidation curves and (b) the corresponding loading efficiency (for permanent compaction) of the wedge sediment (black curve) and the oceanic slab (blue curve); these are described by equation 4. In panel (a), color coded symbols represent laboratory and field measurements for sediments or sedimentary rocks from multiple sites, including Nankai off‐Kumano (Kitajima & Saffer, 2012, 2014), Nankai off‐Muroto (Saffer, 2003), Barbados (Saffer, 2003), and Nias Sumatra (Bray & Karig, 1985). The four numerical digits, sometimes preceded by a “C”, denote Integrated Ocean Drilling Program (IODP) sites.…”

“…To constrain the stress‐dependent α b for our simulations, we compile the results of various laboratory consolidation experiments, along with field data that define porosity in exhumed subduction systems (Bray & Karig, 1985; Kitajima & Saffer, 2012, 2014; Saffer, 2003) (Figure 3). In general, the consolidation behavior of fine‐grained marine sediments can be described by an exponential decay of porosity ( n ) with effective mean stress ( ) (Long et al, 2011; Skarbek & Saffer, 2009): where n 0 is the porosity at zero effective stress (i.e., at the seafloor; set at n 0 = 0.6 in our model), σ ref is a reference stress (set as 18 MPa for wedge sediments), and n ir is an irreducible porosity (set as 0.02), representing the lower limit that porosity will approach.…”

“…Results shown as smooth curves are from Hüpers and Kopf (2012). Discrete data (shown as circles or dots) are from Kitajima and Saffer (2014). (b) Permeability of faults is enhanced by a factor that linearly depends on the shear strain above a certain strain threshold (0.2 in REF) and below an upper limit (2 in REF).…”

“…We thank Laura Wallace, Shuoshuo Han, Harm van Avendonk, Nathan Bangs, Donna Shillington, Kelin Wang, Earl Davis, Xiang Gao, Donald Fisher, Shuichi Kodaira, Dan Bassett, Harold Tobin, Rachel Lauer, and Patrick Fulton for helpful discussions. Data used in Figures 3, 4, and 10 are available in Bray and Karig (1985), Neuzil (1994), Moore et al (1995), Foucher et al (1997), Becker et al (1997), Screaton et al (2002), Saffer (2003), Tsuji et al (2006), Tobin and Saffer (2009), Expedition 322 Scientists (2010), Expedition 333 Scientists (2012), Hüpers and Kopf (2012), Kitajima and Saffer (2012, 2014), and Li et al (2018).…”

Linking Faults, Fractures, and Clay Mineral Occurrence With Fluid Transport in the Accretionary Prism of the Nankai Trough, Japan

“…The sediments in the Nankai Trough accretionary prism predominantly deform due to subduction, abrasion and sediment consolidation, whereby fluid pressure rises and fluids are expelled due to fluid-rock interaction processes (Kitajima & Saffer, 2014). These fluids are able to affect the dynamic processes in the accretionary wedge while they become trapped at depth (Kastner et al, 1991;Saffer & Bekins, 1998).…”